The neuropeptide orexin (also known as hypocretin) plays a central role in the regulation of sleep/wake behavior and the pathology of narcolepsy. People with narcolepsy have difficulty maintaining wakefulness and often have intrusions of REM sleep-like phenomena into wakefulness. These individuals often have decreased numbers of orexin neurons and undetectable concentrations of orexin in cerebrospinal fluid, and mice and dogs with mutations in the genes for orexin or its receptors have a phenotype resembling narcolepsy. As orexin appears necessary for the normal maintenance of wakefulness and suppression of REM sleep, it is critical that we identify the factors that influence the activity of orexin neurons. We propose a model in which orexin neurons promote wakefulness and inhibit REM sleep by activating aminergic arousal regions, and, in turn, the orexin neurons are activated by these aminergic regions. Thus, orexin neurons should be active during wakefulness, and this orexin neuron activity should further increase the activity of aminergic arousal regions, thereby promoting and stabilizing wakefulness. By increasing aminergic activity, orexin neurons also should inhibit REM sleep. In the absence of this positive feedback, the amount of wakefulness should be reduced, bouts of wakefulness should be shorter, and the amount of REM sleep should be increased. Wakefulness and REM sleep are tightly regulated by circadian factors, and we also propose that orexin neurons are influenced by circadian factors, thus contributing to the circadian regulation of sleep/wake behavior. In rats, this circadian signal would facilitate the activation of orexin neurons during the night phase, thereby promoting wakefulness and suppressing REM. In the absence of orexin, the circadian influence on wakefulness and REM should be lost. Our proposed experiments will test the roles of these aminergic and circadian influences. We will first determine whether orexin neurons are active during wakefulness and whether this activation is dependent upon circadian phase. We will use the expression of Fos protein and c-fos mRNA in orexin neurons as well as the concentration of orexin in CSF as indicators of orexin neuron activity, and we will correlate these measures with sleep/wake behavior. We then will determine whether orexin mediates circadian influences on sleep/wake architecture by studying rodents in a light/dark cycle or in constant darkness; these experiments will use two models of orexin deficiency: orexin knockout mice and transgenic rats with an acquired loss of orexin neurons similar to human narcolepsy. Next, we will determine whether orexin neurons are innervated by aminergic arousal regions and express excitatory amine receptors. To test the importance of these aminergic afferents, we will study the response to amphetamine of orexin knockout mice and transgenic rats lacking orexin neurons. By investigating these circadian and aminergic influences on orexin neurons, we will gain critical insights into the normal function of orexin neurons that should provide new perspectives on normal behavioral state control and the neurobiology of narcolepsy.